A power supply or power converter converts one form and voltage of electrical power to another desired form and voltage. AC-to-DC power supplies convert alternating current voltage, for example 115 or 230 volt alternating current (AC) supplied by a utility company, to a regulated direct current (DC) voltage. DC-to-DC power supplies convert DC voltage at one level, for example 400V, to another DC voltage, for example 12V.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. An SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage.
A SMPS uses a high frequency switch, a transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter. SMPSs can be used to step-down a supply voltage as well as provide a step-up function and an inverted output function. An SMPS converts an input voltage level to another level by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The storage may be in either electromagnetic components, such as inductors and/or transformers, or electrostatic components, such as capacitors.
With the introduction of high speed composite semiconductor switches, such as metal oxide semiconductor field effect transistor (MOSFET) switches operated by pulse width modulation (PWM), recent SMPS topologies are now capable of operation at greatly increased switching frequencies, such as, for example, up to 1.0 MHz. However, to be able to provide very low output power with a SMPS, it is necessary to minimize the switching frequency and/or the amount of energy that is transferred to the secondary side of the power supply with each pulse. Very low switching frequency has the disadvantage that voltage sensing via a winding of the transformer is very slow. Conventional voltage sensing is done with a sample and hold element that samples the voltage at a primary side transformer winding while current flows in the secondary winding. This is done when the main switch is in an OFF state. The sensed voltage is used to determine the output voltage. Conventional methods control the on time of the main switch without real-time feedback, which prevents the on time of a present pulse from being influenced by characteristics of the same present pulse. Instead, conventional means provide delayed feedback where characteristics related to a preceding pulse influence the on time of a present pulse.
The power that is transferred to the secondary side is P=Wp*fs, where Wp is the energy transferred with each pulse and fs is the switching frequency of the main switch. To minimize the no load power the transferred power P must be as small as possible because it must be consumed by a base load. Otherwise the output voltage rises if no load is connected. The pulse must have a minimum pulse width to ensure that some energy is transferred to the secondary side. To minimize the transferred power P it is necessary to decrease the energy Wp that is transferred with each pulse and the switching frequency fs.
It is very difficult to control the on time precisely enough such that the energy transfer is minimized, but still big enough to have a small amount of energy transferred to the secondary side without direct feedback. If the pulse is too small, all of the energy is lost in parasitic elements. A small variation of the pulse width has quite big influence on the amount of energy that is transferred to the secondary side.